In general, a coating composition (coating) should exhibit superior adhesivity to an object, such as metal or a polymer film, to which the coating composition is applied, coating hardness, contamination resistance, hydrolytic resistance and the like. Although excellent in hardness, contamination resistance, hydrolytic resistance or the like, a coating composition using a binder such as an alkyd resin, vinyl resin or silicone resin is greatly restrictive and difficult in practical application, because processability thereof is poor. In order to overcome these disadvantages, use of a high-molecular weight polyester resin as a coating binder is considered. However, high-molecular weight polyester resin binders having a linear structure have superior processability, but have disadvantages of poor coating hardness, contamination resistance, hydrolytic resistance and the like.
In order to improve physical properties of polyester resin binders, Japanese Patent Publication Hei. 2-209979 discloses a method of using an alkylene oxide group adduct of bisphenol-A. This method increases polyester resin molecular weight and thereby enhances processability to some extent and improves adhesivity to metal materials and hardness, but has disadvantages of deteriorating UV stability due to increase in ether bonds caused by excessive introduction of bisphenol-A structure, lowering weather resistance of the coating films, and having a low coating film hardness due to a resin glass transition temperature of 70° C. or less. In addition, Japanese Patent Publication Sho. 62-21830 discloses a polyester resin comprising terephthalic acid and alkylene glycol as main components and the resin is excellent in processability, but is disadvantageously not preferable in terms of hardness and contamination resistance. Japanese Patent Publication Hei 7-18169 discloses a polyester resin prepared using isophthalic acid and anhydrous phthalic acid as main acid components and using a C5 to C6 alcohol component such as neopentyl glycol or 1,6-hexanediol. This method aims to satisfy both hardness and processability, but has limitations in improving coating hardness since inherent resin hardness is low and has a disadvantage of sharp decrease in processability when a tri-functional raw material is used to overcome such limitations.
Meanwhile, generally used resins such as polyester, polycarbonate, polystyrene, polyethylene, polypropylene and styrene-acryl copolymers are obtained from fossil resources such as petroleum. Recently, in response to an increase in carbon dioxide in air due to depletion of fossil resources and mass consumption of petroleum resources and thus global warming problems, attempts to reduce consumption of fossil fuels are underway. For example, when resins derived from plants, which grow while emitting carbon dioxide to air, are used, carbon dioxide is circulated in the environment, helping to solve global warming and petroleum resource depletion problems.
Regarding such an environment-circulating resin (polymer), an increase in biomass raw material content (organism total amount or bio-content) is preferred. Biomass means an organic living system such as a plant, a microorganism, a fungi and an animal which gets a solar energy. Biomass raw materials include environment-circulating resources derived from plants including starch-based resources such as cereals and potatoes, cellulose-based resources such as herbs, trees, rice straw and bran, and saccharine-based resources such as sugar cane and sugar beet, environment-circulating resources derived from animals such as livestock excretions and carcasses, and microbial cells, and organic waste derived from these resources such as paper and food waste. Biomass raw material is recyclable, is not depleted unlike fossil fuels and is eco-friendly since carbon dioxide emitted to air by combustion is also circulated in a natural state. Such biomass raw material can be used as an energy source or a raw material for various synthetic products and can be used as an alternative to conventional petrochemical products by applying a biological or chemical technique to the biomass raw material.
Accordingly, recently, a method of improving heat resistance of a polyester resin using isosorbide which is a kind of biomass-derived compound represented by the following Formula 1 and derived from starch as a co-monomer of the polyester resin has been developed.

As isosorbide is a secondary alcohol and thus has a low reactivity, isosorbide is known to be difficult to prepare high-viscosity polyesters used for production of sheets or bottles. However, U.S. Pat. No. 5,959,066 discloses a method for preparing polyester having an intrinsic viscosity of 0.35 dig or more by melt polymerization using various diols comprising terephthalic acid and isosorbide. Polyester resins having an intrinsic viscosity of 0.35 dig or more are used for optical products and coating applications, polyester resins having an intrinsic viscosity of 0.4 dig or more are used for CD, and polyester resins having an intrinsic viscosity of 0.5 dig or more are used for bottles, films, sheets, injection applications and the like. In addition, U.S. Pat. No. 6,063,464 discloses a method for preparing polyester having an intrinsic viscosity of 0.15 dig or more by melt polymerization using a glycol component comprising isosorbide. In the patents described above, a total content of the biomass-derived compound is maintained at a low level although polyester is prepared using ordinary polyester polymerization raw materials, methods and catalysts, and isosorbide.